In a CRT (Cathode-Ray Tube), an electron beam is deflected by a deflecting magnetic field and is irradiated onto its fluorescent surface, to display an image on a screen. The radius of the fluorescent surface of the CRT is larger than a radius from a deflecting center point of the electron beam to the fluorescent surface. Accordingly, the amount of movement of the electron beam in the periphery of the screen relative to the same amount of deflection is larger than the amount of movement of the electron beam at the center of the screen. As a result, when cross-hatched patterns which are inherently equally spaced are displayed on the screen, there occurs such a phenomenon that the spacing between the cross-hatched patterns is widened toward the periphery of the screen from the center thereof.
Such a phenomenon occurs in both the horizontal direction and the vertical direction of the screen. Since the difference in deflection in the horizontal direction is larger than that in the vertical direction, however, the distortion in the image significantly appears. Such distortion in the image is called east-west pincushion distortion. Therefore, a deflection current is generally caused to flow such that the amount of deflection in the periphery of the screen is reduced to correct the east-west pincushion distortion.
When the east-west pincushion distortion is corrected such that vertical lines at right and left ends of the image on the screen are straight lines, however, there occurs such a phenomenon that vertical lines to be straight lines in intermediate portions between the center and the right end and between the center and the left end are curved inward. Such a phenomenon is called inner pincushion distortion.
As the CRT is thinned and flattened, the inner pincushion distortion is increased. In order to correct the inner pincushion distortion thus increased, the amount of the correction must be increased.
In the CRT, the electron beam is deflected by the deflecting magnetic field to display the image on the screen, as described above. In this case, in order to generate the deflecting magnetic field, a deflection current of several App (ampere peak-peak) is caused to flow in a deflection yoke in a deflection circuit. When the inner pincushion distortion is corrected in the deflection circuit, the deflection current is modulated. However, the amount of the current is large. Accordingly, the larger the amount of correction becomes, the higher consumed power becomes in units of several watts (W). Therefore, it is difficult to correct the inner pincushion distortion by adjusting the deflection current in the deflection circuit while restraining the consumed power.
When the inner pincushion distortion is corrected in the deflection circuit, the circuit configuration becomes complicated, thereby preventing the cost from being reduced.